The present invention relates to a circuit arrangement for protecting the electronic interface circuit of subscriber line circuits in a digital time multiplex telecommunications nework which are coupled in groups to a supply battery for providing subscriber current supply, using thyristors as protective elements which provide a low impedance shunt path for excessive voltages on the conductors of the subscriber line coupled to the subscriber line circuit.
The requirement that voltages greater than the supply voltage do not reach components of the interface circuit can be provided for if the supply battery voltage is applied to the control electrode of the thyristors used a protective elements.
Over-voltages due to the effect of lightning on the subscriber lines are primarily the over-voltages to be considered against which such a protective circuit should be effective. A plurality of conductor pairs is always affected in a lightning strike, so that the thyristor protective elements must be simultaneously triggered to be in conductive condition in a plurality of conductor pairs, with the result that the battery, which may comprise an electronic switching regulator, is not able to supply the sum of the trigger, or firing, currents where the trigger current is in the order of 50-250 mA per thyristor, which signifies that proper response of the protective elements for all affected subscriber lines is no longer assured.